remove const_fn from some error_code descriptions

Author: RalfJung

compiler/rustc_error_codes/src/error_codes/E0379.md

@@ -3,8 +3,6 @@ A trait method was declared const.
 Erroneous code example:
 
 ```compile_fail,E0379
-#![feature(const_fn)]
-
 trait Foo {
     const fn bar() -> u32; // error!
 }

compiler/rustc_error_codes/src/error_codes/E0764.md

@@ -3,7 +3,6 @@ A mutable reference was used in a constant.
 Erroneous code example:
 
 ```compile_fail,E0764
-#![feature(const_fn)]
 #![feature(const_mut_refs)]
 
 fn main() {
@@ -27,7 +26,6 @@ Remember: you cannot use a function call inside a constant or static. However,
 you can totally use it in constant functions:
 
 ```
-#![feature(const_fn)]
 #![feature(const_mut_refs)]
 
 const fn foo(x: usize) -> usize {